1. Field of the Invention
The present invention relates to a method of producing a circuit board, in which a conductive paste in an insulating substrate is sintered to form a circuit conductor.
2. Description of Related Arts U.S. Pat. No. 6,228,467 (corresponding to JP-A-2000-38464) and JP-A-2002-299833 disclose methods for producing a circuit board, especially a multilayer circuit board having a multilayer wiring pattern. In this method, a conductive paste in an insulating substrate is sintered to form a circuit conductor.
FIGS. 4A-4I, 5A and 5B are cross-sectional views showing a method for producing a multilayer circuit board 90 similar to that in the U.S. Pat. No. 6,228,467.
Firstly, as shown in FIG. 4A, a resin film 20 including a thermoplastic resin film 21 and a copper foil 22 is prepared. The resin film 20 is a laminated film with the thermoplastic resin film 21 and the copper foil 22.
Then, as shown in FIG. 4B, a resist coat 23 is formed on the copper foil 22. As shown in FIG. 4C, a mask pattern 23a is formed in a photo process.
Next, as shown in FIG. 4D, an etching of the copper foil 22 is performed by using the mask pattern 23a. Thereafter, as shown in FIG. 4E, the mask pattern 23a is removed so as to form a wiring pattern 22a made of the copper foil 22.
Next, as shown in FIG. 4F, a protecting film 24 is laminated on the other face of the thermoplastic resin film 21.
Next, as shown in FIG. 4G, a laser is irradiated from the opposite face of the wiring pattern 22a so that a based hole H is formed. The wiring pattern 22a is the bottom of the based hole H.
Next, as shown in FIG. 4H, the based hole H is filled with a conductive paste 11 which is a conductive filler made of Ag and Sn. Then, as shown in FIG. 4I, the protecting film 24 is removed. Accordingly, the resin film 20 before a multilayer is prepared.
Next, the resin films 20a-20f are stacked in a direction as shown in FIG. 5A. Next, the stacked resin films 20a-20f are inserted between a pair of thermal pressing boards (not shown), and pressed with heat. An adherence protecting film is provided between the resin films 20a-20f and the thermal pressing boards.
The thermoplastic resin films 21 provided by the resin films 20a-20f are softened by the thermal pressing and integrated, as shown in FIG. 5B. In addition, the conductive paste 11 is heated and sintered to form a connecting conductor, which connects the wiring patterns 22a, so that the multilayer circuit board 90 is produced.
In the above-described conventional method, a photolithography method is used. The photolithography method has many processes and causes an increase in cost for producing the circuit board 90. Moreover, a wet etching is used for the etching of the copper foil 22. The wet etching has many restrictions due to a submerged process in the production of the circuit board 90.
By contrast, in the method for producing the circuit board disclosed in JP-A-2002-299833, a conductive paste including metal particles is used. The diameter of the particles is 1-100 nm on average. A wiring pattern is formed by a screen printing or an ink jet using the conductive paste. The metal particle is sintered at the temperature equal to or less than 250° C. Furthermore, for forming a multilayer wiring pattern, each wiring pattern is build up sequentially.
However, the surfaces of the metal particles used for the conductive paste are easy to be oxidized. Even when the metal particles are sintered at a low temperature, a resistance of the wiring pattern is easy to increase. The multilayer by the build up method has many processes and causes an increase in cost for producing the circuit board.